Conventional power wheelchairs, while vastly improving the quality of life for individuals with restricted mobility, have a number of deficiencies that limit their maneuverability and stability. U.S. Pat. No. 3,955,639 issued to Cragg on May 11, 1976, for example, discloses a typical conventional power wheelchair that employs dual electric motors coupled to fixed rear wheels and freely castering front wheels. Steering is accomplished by independently controlling motor velocity and direction of the dual electric motors to vary the force applied to the fixed rear wheels. The freely castering front wheels help to provide a small overall turning radius for the wheelchair. The use of the drive motors for both propulsion and steering in combination with the freely castering front wheels, however, results in poor directional tracking. In addition, the freely castering front wheels may also make the starting of movement difficult if the wheels are misaligned from the intended direction of motion, thereby making some maneuvers practically impossible in small confined spaces. Conventional wheelchairs also generally employ a rigid frame which can cause a wheel to loose ground contact on uneven surfaces. The loss of ground contact can cause instability that might result in the wheelchair tipping over. A more detailed discussion of conventional power wheelchair dynamics and the problems associated therewith is provided in an article entitled "Dynamic Modeling of an Electric Wheelchair", B. W. Johnson & J. H. Aylor, IEEE Transactions on Industry Applications, Vol 1A-21, No. 5, 1985.
Three-wheel personal mobility vehicles or scooters have more recently become a popular replacement for conventional wheelchairs for many individuals with limited mobility. See, for example, U.S. Pat. No. 4,570,739 issued to Kramer on Feb. 18, 1986. The personal mobility vehicles or scooters generally resemble downsized golf carts with a single manually steerable front wheel and two fixed rear wheels. The track of the rear wheels of such vehicles is generally narrower and the wheelbase is generally longer than that of conventional wheelchairs. While such vehicles provide some advantages over conventional wheelchairs, the scooters do not maneuver as well as the typical wheelchair due to their longer wheelbase. The scooters also have disadvantages of their own, including susceptibility to side-to-side tipover due to the small diameter wheels that are generally employed in their manufacture. In addition, the difficulty in entering and exiting the scooter and the lack of power steering has limited the use of scooters to less impaired individuals. In fact, many individuals with progressive diseases that start out using a scooter have to resort to using a conventional power wheelchair--with all of its inherent disadvantages--in the latter stages of their disease, due to the limitations of the scooters discussed above.
The undesirable tracking and stability characteristics of conventional wheelchairs and scooters is further compounded by the fact that such devices are usually designed to be lightweight in order to permit easy handling and storage. As a result, the mass of the occupant equals or exceeds the mass of the wheelchair or scooter in most cases. Thus, movements by the occupant that shift the mass center of the occupant can greatly impact the stability and directional tracking of the wheelchair or scooter.
In view of the above, it is an object of the present invention to provide a power wheelchair that overcomes the disadvantages associated with conventional wheelchairs and scooters. It is a further object of the invention to provide a power wheelchair that compensates for the movement of an occupant, such that the movement of an occupant does not adversely impact the stability and directional tracking of the wheelchair.